Steelmaker Tenaris (TENA.BA) (TS.N), the world’s leading producer of seamless steel tubes for the energy industry, rallied on stronger oil prices to 42.35 pesos a share. U.S. crude futures ended lower after rising earlier in the day.

The benchmark MerVal index .MERV closed 5.42 percent higher at 1,214.28 for a 17 percent gain over the last six trading days. Tenaris accounts for more than one-third of the MerVal’s weighting.

“The rise in crude oil pushed Tenaris and Brazil’s Petrobras higher, and these stocks accounted for 60 percent of total trade volume,” said Mariano Tavelli of brokerage Tavelli & Co.

Volume on the broad market was a moderate $21.5 million. Of active shares, 55 rose, 11 fell and 14 were unchanged.

Some bonds have gained as much as 25 percent in recent weeks on persistent rumors of a debt swap involving “guaranteed loans” issued mainly to banks before the country’s 2002 default and, possibly, Boden 2012s ARBODEN12=RASL ARBODEN12D=RASL, in a bid to lengthen debt maturities.

The government has made no announcement, however.

On the foreign exchange market, the peso lost ground as the central bank bought dollars, traders said.

In informal trade between foreign exchange houses ARSB= as measured by Reuters, the peso shed 0.71 percent to 3.535/3.54 to the dollar.

In formal interbank trade ARS=RASL, where the central bank intervenes nearly every day, the local currency dipped 0.07 percent to end at 3.4525/3.455 per dollar. (Reporting by Walter Bianchi and Jorge Otaola; Writing by Hilary Burke; Editing by James Dalgleish)